1. Field of the Invention
The present invention generally relates to the field of microelectronic transistor devices, and more specifically to a method of fabricating an improved heterostructure which is especially suited for an inverted modulation-doped, or high-electron-mobility transistor (HEMT).
2. Description of the Related Art
Gallium indium arsenide (GaInAs)/aluminum indium arsenide (AlInAs) based HEMTs fabricated on indium phosphide (InP) substrates currently exhibit the highest current gain cut-off frequencies, highest maximum frequencies of oscillation and lowest noise figure of any three terminal device. These HEMTs include a modulation-doped heterojunction at which an n-doped A1InAs donor layer provides electrons to form a two-dimensional electron gas (2DEG) in an undoped GaInAs channel layer. A1InAs is a wide bandgap material, whereas GaInAs is a narrow bandgap material.
In a conventional HEMT, the donor layer is formed above the channel layer, whereas in an inverted HEMT, the donor layer is formed below the channel layer. A double-doped HEMT structure is also known, including donor layers above and below the channel layer. The high performance of these HEMTs is due, in part, to the relatively high 2DEG concentration (greater than 3.times.10.sup.12 /cm.sup.2) produced at the heterojunction, as well as the high mobility of electrons (greater than 10.sup.4 cm.sup.2 /Vs, lattice matched) in the undoped GaInAs channel.
HEMTs are preferably fabricated using molecular beam epitaxy (MBE). The process has been improved to reduce the substrate temperature from the previous value of 600.degree. C. to 500.degree. C. to reduce dopant diffusion in gallium arsenide (GaAs) based HEMTs, such as described in an article entitled "High performance inverted HEMT and its application to LSI", by S. Nishi et al, Inst. Phys. Conf. Ser. no. 83, 1987, pp. 515-520.
A1InAs/GaInAs HEMTs require an n-type dopant, usually silicon (Si), in the AlInAs donor layer. The Si may be distributed through the thickness of the donor layer, or may include a substantially planar layer of Si buried in an undoped AlInAs layer (delta or planar doping). Under normal MBE growth conditions at 500.degree. C. or above, A1InAs/GaInAs HEMTs suffer from extrinsic degradation due to surface segregation of Si in A1InAs.
In normal HEMT structures in which the doping is above the AlInAs/GaInAs heterojunction, the segregation of Si toward the surface of the epitaxial layer leads to a degraded transfer efficiency of electrons into the 2DEG. To compensate, higher doping concentrations are then required to achieve a given 2DEG concentration, which in turn can degrade device performance by increasing the gate to drain capacitance.
In inverted HEMTs, in which the doping is typically less than 100 Angstroms below the AlInAs/GaInAs heterojunction, the segregation of Si leads to dramatically reduced mobility of the 2DEG due to ionized impurity scattering, and consequently poor conductivity in inverted as well as double-doped HEMTs. This is much more deleterious than the problems caused by surface segregation in the conventional HEMT structure.
The use of spacer layers grown at low temperatures on the order of 300.degree. C. to slow beryllium (Be) diffusion out of GaInAs P base layers in heterojunction bipolar transistors (HBTs) is known, such as described in an article entitled "IMPROVED HIGH FREQUENCY PERFORMANCE OF A1InAs/GaInAs HBTs THROUGH USE OF LOW TEMPERATURE GaInAs", by W. Stanchina et al, in InP and Related Compounds Conference Proceedings, Denver 1990, pp. 13-16.
However, Si is a relatively large atom, and does not diffuse in A1InAs as does a relatively small atom such as Be in GaInAs. In surface segregation, the large Si atoms "float" toward the surface of an AlInAs layer at 500.degree. C. or above during MBE growth, whereas in diffusion the relatively small Be atoms migrate throughout the layer.
As discussed in an article entitled "Si dopant migration and the AlGaAs/GaAs inverted interface", by L. Pfeiffer et al, Appl. Phys. Lett. Vol. 58, No. 20, May 20, 1991, pp. 2258-2260, low temperature MBE would be expected to produce non-optimal growth conditions in AlGaAs which would result in new problems such as deviations from stoichiometry, increases in impurity sticking coefficients during growth, and increased alGaAs roughness. The solution described by Pfeiffer involves the formation of a spacer layer at the normal growth temperature of 640.degree. C. which is much thicker (800 Angstroms) than conventional spacers (400 Angstroms).